Television system



Sept. 2, 1941'. A. n BLUM'LEIN 7 2,254,204

TELEVIS ION SYSTEM Filed May 11, 1937 2 Sheets-Sheet l AAlAA vvvvv W H H4. INVENTOR w ALAN 0019511114151 1 WW ATTORNEY Sept. 2, 1941. A. D.BLUMLEIN TELEVIISIION SYSTEM i -2 Sheets-Sheet 2 Filed May 11, 1937patented Sept. 2, 1941 2,254,204 TELEVISION SYSTEM Alan Dower Blumlein,Ealing, London, England,

assignor to Electric & Musical Industries Limited, Hayes, Middlesex,England, a company of Great Britain Application May 11, 1937, Serial No.141,917 In Great Britain May 15, 1936 13 Claims.

This invention relates to television systems and has particularreference to methods of obtaining automatic gain or volume control.

When television signals are to be transmitted to or from moving objectssuch as aeroplanes, a gain control efiect which will act rapidly isrequired, and while the invention has particular application toair-craft television systems and also to other light-weight equipment,it may also be employed in a general way, in wireless or line systemsemploying, for example, co-axial conductors.

The object of the present invention is to provide a method of producinga television waveform from which automatic gain control potentials maybe obtained and receiver circuits responsive to such a waveform.

According to the present invention a waveform representing a scene orpicture to be transmitted by a television system comprises portionsrepresenting successive lines of the scene or picture, interrupted bysynchronising signals which take the form of double pulses a portion ofwhich extends in one direction followed by a portion extending in theopposite direction, both portions having amplitudes lying outside thenormal amplitude range occupied by signals representing visionamplitudes. The waveform may constitute a modulating waveform for acarrier Wave of a transmitting system, or it may constitute the envelopeof a carrier wave which is modulated by successive signals and line andframe synchronising signals, said line synchronising signals beingconstituted by a portion of said double pulses, the other portion ofsaid double pulses constituting separate signals from which gaincontrolling potentials may be derived, said line synchronising signalsmodulating the carrier wave in the blacker-than black sense and beingsucceeded immediately by said separate signals which modulate thecarrier wave in the whiterthan-white sense.

' A circuit arrangement for producing a wave form in accordance with theinvention may include a valve to which picture signals interrupted atthe line and frame frequencies are applied, said valve having its outputcombined with a second valve, to which a set of pulses at the linefrequency are applied, the combined output being passed to athird'valve, the output from which is combined with the output from afourth valve supplied with a second set of pulses at the line frequency,but delayed with respect to saidfirst set of pulses.

A particular form of television receiver adapted to be responsive to acarrier wave modulated by a wave form produced in accordance with theinvention, includes high or intermediate frequency amplifying stages, apicture signal rectifierrand a rectifier which providesgain controllingpotentials for said high frequency amplifying stages, said gaincontrolling potentials being derived from one portion of the doublepulses in the wave form. Part of the output from one of the high orintermediate frequency amplifying stages is fed to a further amplifyingstage, and another part is fed to a circuit arrangement including arectifier which provides automatic gain control potentials, the outputof said further amplifying stage being maintained constant by applyingto it automatic gain control potentials of selected value. i

In order that the invention may be more clear- 1y understood and readilycarried into effect, a methodoi obtaining automatic gain control inaccordance therewith will now be more fully described with reference tothe accompanying drawings in which- Fig. 1 shows an alternating currentWave form including picture signals, line and frame synchronisingsignals, and additional signal pulses usedfor providing automatic gaincontrol,

Fig. 2 shows a circuit arrangement for producing-the Wave form shown inFig. 1,

Figs. 3 and 4 show respectively sets of signal pulses which are appliedto the circuit of Fig. 2 for the production of the wave form shown inFig. 1, Y

Fig. 5 shows a form of receiving circuit suitable for use with the waveform shown in Fig. 1, and Fig. 6 is a circuit arrangement illustrating adetail in the operation of a receiver.

Referring to Fig. l of the drawings, the point i represents zero, thepoint 2 represents the mean value of picture signal current, While 3represents the'peak or maximum of modulating signals- The amplitudes ofthe picture signals are limited to a range within the zero and peakpoints so that maximum white in the picture does not reach the peakvalue of the modulating signals, and black does not reach zero value.The peak points are thus values representing whiter than white signalsand the zero points represent blackerthan-black signals. The diagramshows several lines 4 each having a duration of micro-seconds togetherwith line synchronising signals 5 obtainedby reducing the carrier waveto substantially zero for 10 micro-seconds, and a frame synchronisingsignal 6 extending over ten complete line periods between the points Aand B. As will be seen, the line synchronising signals extend in theblacker-than-black direction and the frame synchronising signal 6extends in the whiter-than-white direction.

Immediately following the line synchronising signals is an impulse Iwhich extends in the whiter-than-white direction and is equal induration to a line synchronising signal, that is a period ofmicro-seconds. These impulses are used to provide gain controllingpotentials at a receiver in the manner to be described with reference toFig. 5 of the drawings. The gain controlling potentials obtalned areproportional to the height of the signals '1 and also to the framesynchronising signals 6 which, as will be seen in Fig. 1, areinterrupted by the line synchronising signals continuing during a framesynchronising signal.

The wave form shown in Fig. 1 may be generated by the circuitarrangement shown in Fig. 2 of the drawings. Incoming picture signalsare applied to the valve 8 through a coupling condenser 9, the picturesignals being free of the pulses 5, 6 and 1 shown in Fig. 1. Theintervals between the signals representing successive lines andsuccessive frames may contain spurious signals such as are produced forexample by an electrical scanning tube. Signal pulses Ill of the formshown in Fig. 3 are applied to the grid of a valve II through a couplingcondenser [2. These signals are mixed in an anode resistance I3 commonto the valves 8 and l I and are passed to a valve l4 through a couplingcondenser IS, the valve !4 having a comparatively high resistance it inits cathode lead. The efiect of the insertion of the resistance l6 is togive the characteristic curve of valve [4 a long linear portion .1

with comparatively little curvature at the region of anode currentcut-off. The valve I4 is so biassed as to cut off the pulses added byvalve 1 l and to produce fiat tops on the pulses 1 shown in Fig. 1. Theline pulses ID shown in Fig. 3 are of r slightly longer duration thanthose obtained in the final wave form, and they occur towards the end ofthe interval between lines. A further valve ll serves to mix with theoutput of valve 14 pulses ll! of the form shown in Fig. 4. The pulses 18are applied to the grid of valve I! through a coupling condenser N2, theoutputs of valves, and I! being combined in a common anode resistance 9.The pulses I8 occur at line frequency only and end slightly after thebeginning of the pulses l0 shown in Fig. 3. The overlapping of thepulses is and I8 assist in producing a clean transition from one pulseto the other in the final wave form.

The mixed output from valves l4 and l! is passed through a couplingcondenser 20 to the grid of a valve 2i which also has a comparativelyhigh resistance 22 in its cathode lead. The valve 2| tends to cut offthe tops of the pulses l8 shown in Fig. 3, thus giving in its anodecircuit a wave form of substantially the shape shown in Fig. 1. Thebeginning and end of the pulses 5 are produced by the pulses [8 shown inFig. 3, and the pulses l, with the exception of the portions neutralisedby the over-lapping portions of the pulses it are formed by the pulsesl0 shown in Fig. 3.

The resistances l6 and 22 in the cathode leads of valves l4 and '2!respectively, serve to depreciate the gain of these valves so that for asmall current the characteristic is sensibly linear and a sharp cut-offis obtained at the bottom of the characteristic curves.

The long pulse I01 shown in Fig. 3 produces the fiat top framesynchronising signal 6 shown in Fig. 1, this long frame pulse beingdivided upby the pulses I8 in Fig. 4, to produce the waveform shown inFig. 1.

The low frequency waveform shown in Fig. 1 is used to modulate a carrierwave generated by transmitting equipment not shown in the drawings. Themethod of modulation is preferably such that the level I in Fig. 1represents zero carrier and the level 3 represents carrier maximum. Theinverse method of modulation may be adopted and again it is notnecessary that the peaks I and 3 should represent zero or maximumcarrier respectively, but they may represent intermediate values.

Fig. 4 shows diagrammatically the portions of a television receiveradapted to utilise the automatic gain control effect provided by thewave form shown in Fig. 1. Five stages of a receiver are represented bythe block diagrams 22, 23, 24, 25 and 2B. The output from the stage 25is passed to an automatic gain control rectifying arrangement containedwithin the dotted rectangle 2'1. The signals fed from the stage 25 arerectified by a diode 28 in conjunction with condensers 29 and 3! ofsmall value, constituting with an inductance 3|, a radio frequencyfilter. The time constants of the condensers 29 and 30 with reference tothe diode anode resistance is such as to pass the full frequency rangeof the signals or a sufficient frequency range to reproduce the pulses 5and l. The direct current biassing arrangements. are not shown in thedrawings as it is well known, for example, that a low frequency returnmust be provided for the lead connecting stage [4 with the diode 28.

The rectified signals obtained from the diode 23 are passed to anamplifying valve 33, and from thence, through a coupling condenser 34 todiode valves 35 and 36 arranged to operate in push-pull fashion. Thedirect current voltage set up across the diode anode resistance 31 willthen be proportional to the double amplitude of the rectified waveprovided by the anode 28. This double amplitude peak is in effect thedistance between the peaks of successive rectified pulses 5 and 1 shownin Fig. 1. The arrangement is in effect a direct currentre-establi'shment by the condenser 34 and diode 35, together with a peakrectification by the diode 36 co-operating with the resistance 3'! andits shunt condenser 38. The time constant of condenser 34 and the anoderesistance 39 'of valve 33 in series with the resistance of either ofthe diodes 35 and 36, is arranged to be longer than the duration of anypulse including the duration of pulse 6. between any consecutivebreaking pulse 5. In other words, this time constant should be longerthan a line period. The time constant of resistance 31 and condenser 38should be several lines longer, and is the controlling time constant forthe speed of the automatic gain control effect. The voltage across theresistance 31 is fed back through lead 40 to one or more stages 22, 23,24 and 25, and also to the stage 26 which it will be noticed follows thestage 25, from which the signals for providing the gain control effectare tapped out. Gain controlling potentials may thus be applied to oneor more of the stages mentioned.

The control of the stage 26 is so adjusted that for'average receptionconditions the'output'of this stage is substantially constant. Forexample, a change of input of the order of 40 dbs. may produce change ofrectified voltage at the diode 28 and from one to two volts. This mayproduce a change of rectified voltage across the resistance 31 of fromten to twenty volts, and this change of voltage applied to the controlstage 26 must alter its gain by 2:1 so as to maintain substantiallyiconstantoutput. The output from the stage 26 is applied'to a rectifier4| and a filter 42 shown within the dotted rectangle 43, the rectifiedand substantially constant output signals being taken from a terminal44. The adjustment of the stage 26 to convert the variable signals fromthe stage I4 into substantially constant signals may be made by means ofa potentiometer not shown in the drawings which controls the fraction ofthe voltage across the resistance 31 applied to the control grid of theamplifier in stage 26. I

In the case of a given set of reception conditions, a manual control ofadditional negative or positive bias potentials supplied to any of thestages 22 to 26 may be provided, so that the output from rectifier28'may be kept within a certain range for which the correction by thestage 26 is most satisfactory. The correction as shown in stage 26,which as previously pointed out, operates beyond the point from whichthe signals providing the automatic gain control efiect are tapped out,may be omitted, or it may be applied in one or more low frequency orhigh frequency stages, or both.

A rapidly acting gain control effect is obtained by the arrangementdescribed, because the frequency of the'recurrent pulses may be as highas 10,000 per second. The quickness of operation of the automatic gaincontrol effect is controlled .by the time constant of the resistance 31and its associated condenser 38, which may for example, be of the orderof lines. Thus, the time constant of the automatic gain control will beone hundredth of a second, which is Very much shorter than is possiblein normal sound channel automatic gain control systems.

In order that picture-interference may not occur, it is necessary at thereceiver to black out or suppress some of the pulses which occur in' thewhite direction. If as in the case described, the transmission is sentwith a high value of carrier modulation representing white, theautomatic gain control peaks must be blacked out to prevent themappearing as White in the picture. Again, if a low value of carriermodulation represents white in the picture, the line synchronisingsignals must be blacked out.

An alternative form of circuit arrangement for effecting blacking out ofpulses is shown in Fig. 6 of the drawings accompanying thisspecification, In this figure the signals are applied to delay networkrepresented by the block 45, and a tapping is taken from the input tothe delay network to a limiter valve 46 which only passes amplitudes inexcess of the peak white amplitude. The output from the limiter valve 46is passed to a further delay network represented by the block 41, thetime constant of this delay network being double that of the network 45.Signals are taken from the input and output of the delay network 41 andpassed to. the control grids of two valves 48 and 49 respectively, theanode currents of these valves being combined in a common loadresistance 59. The voltage across the load resistance 50 is mixed withthe aid of further amplifying valves 5! and 52 with vision signalsobtained from the output end of the delay network 45. The mixed outputfrom'the valves 48 and 49 is fed through a condenser. 53 to the controlgrid of Valve 5|, and the vision signals from the delay network 45. arefed to the control grid of valve 52 through a coupling condenser 54. Thevalves 5| and ,52 again have a common load resistance 55, the mixedoutput being passed to a cathode ray tube connected to the terminal 56.The mixing of the signals in the valves 5| and- 52 is in such a sensethat the signals obtained from the delay network 41 serve to addeffective black to the vision signals arriving from the delay network45. The arrangement will in effect produce black vision signals in placeof white from just before the white pulse period to just after the whitepulse period, the amount of overlap being the delay of the first delaynetwork 45. Any difference of delay for the direct vision path from thenetwork 45 to the valve 52 and the blacking out path, including thedelay network 41 produced by the inevitable stray capacities andinductances of the amplifying valves, may be corrected by altering thedelay of the network 45 while maintaining the delay of the network 41constant. The high frequency cut-off of the delay network 45 must besufiiciently high to ensure that the vision signals will be transmittedwithout distortion, but it is not necessary for the delay network 41 tobe of the same quality. The period of overlap which may .be of the orderof half micro-second is willcient to ensure complete suppression of thewhite signals.

I claim:

1. A television transmission system including means .for developingvideo signals, means for developing synchronizing signals to betransmitted in the interval between trains of picture signals, means fordeveloping a signal of opposite polarity to the synchronizing signal,means for combining the synchronizing signal and said latter signal soas to'form a signal having two differing components of polarity, saidcombined signal being indicative of the transmission level of thetransmitter, the receiving means including a plurality of amplifyingstages for receiving said transmitted signals, means for developing acorrective signal proportional to the value of the combinedsynchronizing signal and the signal of opposite polarity, means forimpressing said corrective signal onto a plurality of amplifyingstag'es'of said receiver, and additional means for maintaining theoutput of said receiver substantially constant,

2. A television system including means for developing video signals,means for developing synchronizing signals to be transmitted in theinterval between trains of picture signals, means for developing asignal of opposite polarity to the synchronizing signal, means forcombining the synchronizing signal and said latter signal so as to forma signal having two different components of polarity, said combinedsignal being indicative of the transmission level of the transmitter,means for receiving said transmitted signal including a plurality ofamplifying stages, means for rectifying a portion of the output of one01 said stages, means for amplifying said rectified signal, a pair ofuni-directional conductors connected in pushpull relationship forrectifying said rectified signal, and means for utilizing the output ofthe pair of uni-directional conductors to control the gain of aplurality of stages of the receiver.

3. Apparatus in accordance with claim 2 wherein said uni-directionalconductors connected in pushpull comprise a pair of diodes.

4. Apparatus in accordance with claim 2 wherein there is provided inaddition rectifying means connected to the last stage of the receiverfor maintaining the output thereof substantially constant.

5. Television reproducing means for utilizing signals comprisingrecurrent trains of video signals having interposed between said trainsof video signals a synchronizing signal and a signal of oppositepolarity thereto, said synchronizing signal and said signal of oppositepolarity being indicative of the transmission level of the transmitter,said reproducing means including means for receiving said signalscomprising a plurality of amplifying stages, means for developing acorrective signal proportional to the value of the combinedsynchronizing signal and the signal of opposing polarity, means forimpressing said corrective signal onto a plurality of the amplifyingstages, and additional means for maintaining the output of said receiversubstantially constant.

6. Television reproducing means for utilizing signals comprisingrecurrent trains of video signals having interposed between said trainsof video signals a synchronizing signal and a signal of oppositepolarity thereto, said synchronizing signal and said signal of oppositepolarity being indicative of the transmission level of the transmitter,said reproducing means including means for receiving said transmittedsignal including a plurality of amplifying stages, means for rectifyingat least a portion of the output of one of said amplifying stages, apair of uni-directional conductors connected in push-pull relationshipand having at least a portion of the output of said rectifying meansimpressed thereon, and means for utilizing the output of said push-pullconnected uni-directional conductors to control the gain of a pluralityof the stages of the receiver.

'7. Apparatus in accordance with claim 6, wherein said uni-directionalconductors connected in push-pull relationship comprise a pair ofdiodes.

8. Apparatus in accordance with claim 6, wherein there is provided inaddition rectifying means connected to the last stage of the receiverfor maintaining the output thereof substantially constant.

9. Television reproducing means for utilizing signals comprisingrecurrent trains of video signals having interposed between said trainsof video signals a synchronizing signal and a signal of oppositepolarity thereto, said synchronizing signal and said signal of oppositepolarity being lit indicative of the transmission level of thetransmitter, said reproducing means including means for receiving saidsignals, rectifying means for rectifying at least a portion of theoutput of at least one of the stages of said reproducing means, videofrequency filter means connected in the output circuit of saidrectifier, means for impressing at least a portion of the signal passingthrough said filter means onto a second rectifier, a time constantcircuit connected between said filter means and said rectifier, saidtime constant circuit having a period longer than the duration of anypulse passing through said filter means, a third rectifier connected inpush-pull relationship to said second rectifier, a time constant circuitconnected in the output of said third rectifier and having a periodgreater than one line period of the image to be reproduced and less thanone frame thereof, and means for utilizing the output of said thirdrectifier to control the gain of a plurality of stages of thereproducing.

10. Television reproducing means for utilizing signals comprisingrecurrent trains of video signals having interposed between said trainsof video signals a synchronizing signal and a signal of oppositepolarity thereto, said synchronizing signal and said signal of oppositepolarity being indicative of the transmission level of the transmitter,said reproducing means including means for receiving said transmittedsignal including a plurality of amplifying stages, a first means forrectifying at least a portion of the output of one of said amplifyingstages, second rectifying means having at least a portion of the outputof said first rectifying means impressed thereon, a time constantcircuit connected in the output of said second rectifying means, saidtime constant circuit having a period greater than the Period of oneline of the image to be reproduced and less than one frame of saidimage, and means for utilizing the output of said second rectifyingmeans to control the gain of at least one of the stages of saidreceiver.

11. Apparatus in accordance with claim 1, wherein the time constant ofsaid corrective signal developing means is greater than one line of theimage to be reproduced and less than a frame thereof.

12. Apparatus in accordance with claim 5, wherein the time constant ofthe corrective signal developing means is greater than one line of theimage to be reproduced and less than a rame thereof.

13. Apparatus in accordance with claim 6, wherein the time constant ofthe stage containing the rectifying means is greater than one line ofthe image to be reproduced and less than a frame thereof.

ALAN DOWER BLUMLEIN.

